As an electrodeposition paint is excellent in throwing power and capable of forming a coating excellent in such properties as durability, anticorrosivity etc., it has been widely used hitherto in the application fields, in which such properties are required, for example, coating of a car body, coating of electric appliances etc.
In an electrodeposition paint there are often compounded anticorrosive pigments, for example, lead compounds or chromium compounds such as lead chromate, basic lead silicate, strontium chromate etc. in order to further improve it anticorrosivity. These compounds, however, are very harmful substances and their usage is problematic for countermeasures against pollution. Therefore, the usage of zinc phosphate, iron phosphate, aluminium phosphate, calcium phosphate, zinc molybdate, calcium molybdate, zinc oxide, iron oxide, aluminium phosphomolybdate, zinc phosphomolybdate has been hitherto investigated as a non-toxic or low-toxic anticorrosive pigment replacing lead compounds or chromium compounds. These compounds, however, are less anticorrosive than the aforementioned lead compounds or chromium compounds and not practically satisfactory.
It is already known that bismuth is effective as a metal exhibiting as good or even better anticorrosivity than lead compounds or chromium compounds in an electrodeposition paint and, above all, bismuth lactate is particularly effective.
For example, Japanese PCT Publication No. 506870/1995 (=WO93/24578 Pamphlet) discloses a preparation process of a catalyst -containing cationic coating binder containing a cationic coating binder, which can be diluted with water after protonation and can be crosslinked through transesterification an or transamidation and/or transurethanization and/or reaction of terminal double bonds, and bismuth salts of an aliphatic hydroxycarboxylic acid, preferably lactic acid or dimethylolpropionic acid. And said Publication describes that the bismuth salts are obtained by reactin 1 mole of bismuth oxide with 7 moles of an aliphatic hydroxycarboxylic acid. The isolated, dried bismuth salts obtained by this process, however, have a tendency of caking during storage. Moreover the bismuth salts obtained by this process easily produce aggregation or precipitation during the preparation procedures. The tendency is stronger, when less amount of acid is used and it is difficult to use it as a homogeneous aqueous solution. As the amount of acid used for the reaction is more than the amount necessary to neutralize the coating binder in a paint, there rises a problem that the amount of current required during operation of an electrodeposition bath increases considerably.
The present inventors thought that a bismuth salt could be dispersed stably and homogeneously the electrodeposition bath, if a bismuth salt with good water-solubility is used, and such a problem as mentioned above could be solved. As a result of looking for a bismuth salt which does not cake during storage and has a good water-solubility, they found, this time, that bismuth lactate derived from an optically active lactic acid with high L-isomer content and bismuth methoxyacetate are suitable and completed the present invention.